In 1922, Edwin Hubble shocked the scientific community when he discovered that the universe extended beyond the Milky Way. One realization — that hundreds of billions of other galaxies existed — seemed to make our universe even larger. Now, the scientific community may be faced with an even more daunting realization: that we exist in only one of many universes. Hubble once commented, “The history of astronomy is a history of receding horizons.”  As they have been doing since the times of Copernicus and Hubble, our horizons may now recede once again.
This proposition — that our universe is not alone — is known as the multiverse theory.  The origins of the multiverse theory date back to 1980s, when physicists Alan Guth and Andrei Linde started to develop the theory of cosmic inflation. Inflation describes a massive, exponential expansion of the universe within the first 10-35 second of its inception, before slowing down to the more moderate rate of expansion we are experiencing today.  After the predictions of inflation coincided with the then-unexplained patterns of the cosmic microwave background, it seemed as if the scientific community had gained deep insight into the creation of universes.
However, it wasn’t long until the question of whether inflation ever ended was proposed. In 1982, physicist Andrei Linde introduced the model of eternal inflation, claiming inflation actually has no end and continues forever in some parts of the universe. Although the rate of inflation decreases slowly with time, occasional random quantum fluctuations can increase the rate back above a certain threshold. In these areas, inflation continues, and the process repeats itself.  During this never-ending progression, new universes are continuously being spawned.
Even though the idea of the multiverse was introduced by eternal inflation, it was not strongly supported by the scientific community since it was impossible to test experimentally. Just as support for the multiverse theory seemed to dwindle, new developments in string theory rekindled interest. Advancements in string theory revealed that a fundamental unit of the universe, the string, actually exists in nine dimensions.  The shape of these strings would determine how strings vibrate, and would consequently explain the properties of our universe. When solving for the shape of the string that would describe our universe, physicists were shocked to find a number of equally valid solutions on the order of 10500. Although this anomaly cannot be explained with respect to the scope of our own universe, it makes good sense if each one of the 10500 configurations corresponds to its own universe, effectively describing a multiverse.
The possibility that we exist in only one of an infinite amount of universes redefines our sense of uniqueness and individuality. Anything that you can possibly think of doing, you have already done in another universe. Maybe in one universe you became the President of the United States, while in another you never had the chance to attend Princeton. If an infinite number of universes do exist, there are undoubtedly other copies of you doing everything you can imagine.
The possibility that we exist in only one of an infinite amount of universes redefines our sense of uniqueness and individuality.
The validity of the multiverse theory is still a controversial topic of debate within the scientific community. Many physicists such as Alan Guth and Alexander Vilenkin fully support the idea of a multiverse. Others, such as cosmologist and Princeton physics professor Paul Steinhardt, claim the theory should not be given attention due to its inability to be properly tested.  Whether or not the theory of the multiverse becomes confidently accepted as fact depends on if experimental evidence can be found to support it. As of now, such evidence is virtually impossible to come by. So, is the multiverse fact or fiction? Only time will tell.